Generally, the operating principle of BAW components is illustrated in FIG. 1 which shows a bulk wave filter structure: a piezoelectric substrate Spiezo, v is inserted between two metallizations M1 and M2 allowing the propagation of bulk waves.
The following articles recount a complete overview of filters based on resonators of BAW and SAW type: W. Steichen, S. Ballandras, “Composants acoustiques utilisés pour le filtrage: revues de différentes technologies” [Acoustic components used for filtering: reviews of various technologies], Editions de Techniques de l'Ingénieur, E-2000, 31 pages, 2008; R. Aigner, Bringing BAW technology into volume production: the Ten Commandments and the seven deadly sins, Proceedings of the third international symposium on acoustic wave devices for future mobile communication systems (2007). J. Kaitila: Review of wave propagation in BAW thin film devices: progress and prospects, Proceedings of the 2007 IEEE Ultrasonics Symposium. P. Muralt et al.: Is there a better material for thin film BAW applications than AlN, Proceedings of the 2005 IEEE Ultrasonics Symposium.
Bulk wave filters have existed for tens of years at frequencies of a few MHz to a few tens of MHz, using mainly impedance elements or structures with lateral coupling on quartz for narrow-band applications, but their implementation at radiofrequencies dates back only about ten years, following Lakin's pioneering work on the use of piezoelectric layers deposited by cathodic sputtering for such purposes. The company Agilent whose “FBAR filter” (Film Bulk Acoustic Resonator) branch has given rise to the spin-off AVAGO, was the first to develop an RF filter based on impedance elements exploiting thin films of aluminum nitride (AlN), deposited polycrystalline material. Following these technical advances, a large number of academic and industrial researchers have eagerly pursued this track, giving rise to sustained inventive activity during the present decade.
Generally, BAW resonators utilize the thickness-wise resonance of a thin piezoelectric layer which is acoustically isolated from the substrate either by a membrane (FBAR technology used by AVAGO Technologies), or by a Bragg grating (SMR technology used by Infineon). At the present time, the material most widely employed with BAW technology is Aluminum Nitride (AlN), which exhibits the advantage of having piezoelectric coupling coefficients of the order of 6.5%, and also of having low acoustic and dielectric losses, thereby allowing the synthesis of filters exhibiting passbands that are compatible with the specifications stipulated by most telecommunication standards localized between 2 and 4 GHz.
Nonetheless, several problems continue to confront the extremely constraining specifications exhibited by a few frequency bands, such as the DCS standard.
Firstly, the piezoelectric coupling coefficients allowed by AlN do not authorize relative passbands of greater than 6%. Such bandwidths already require the use of electrodes exhibiting a very large acoustic impedance (made of Molybdenum or Tungsten), so as to confine the elastic energy in the piezoelectric layer, and of thicknesses carefully determined so as to maximize their influence on the piezoelectric coupling coefficient of the resonators, as described in the following articles: R. Aigner, Bringing BAW technology into volume production: the Ten Commandments and the seven deadly sins, Proceedings of the third international symposium on acoustic wave devices for future mobile communication systems (2007); J. Kaitila, Review of wave propagation in BAW thin film devices: progress and prospects, Proceedings of the 2007 IEEE Ultrasonics Symposium. At the present time, no credible solutions exist for extending this relative band at constant losses.
Research is being conducted to find other materials exhibiting higher piezoelectric coupling coefficients, but it must be noted that no other material exists which affords low acoustic losses and which is currently able to be deposited reproducibly and uniformly, as described in the article: P. Muralt et al., Is there a better material for thin film BAW applications than AlN, Proceedings of the 2005 IEEE Ultrasonics Symposium.
Conversely, monocrystalline materials such as Lithium Niobate or Lithium Tantalate afford very high electromechanical coupling coefficients, allowing the production of filters exhibiting relative bandwidths of the order of 50%.
Thereafter, standards such as DCS also require both a wide passband and strong rejection of the adjacent standards. To simultaneously meet these two constraints requires the use of resonators possessing very large quality coefficients. Much work has been conducted in the last ten years to improve the acoustic wave confinement properties of resonators (J. Kaitila, Review of wave propagation in BAW thin film devices: progress and prospects, Proceedings of the 2007 IEEE Ultrasonics Symposium). Therefore, the limits imposed by the materials themselves, more than by the structure, are beginning to show up, and it is very probable that poly-crystalline materials will eventually no longer be able to cope with the rise in quality coefficients, especially faced with the rise in frequency of the standards toward 10 GHz. With intrinsic quality coefficients of the order of some ten thousand at frequencies above 1 GHz: D. Gachon et al., Filters using high overtone bulk acoustic resonators on thinned single-crystal piezoelectric layer, presented at the 2008 European Frequency and Time Forum, here again monocrystalline materials present themselves as an interesting solution.
Concerning resonators of FBAR type, Campanella et al. have produced an FBAR resonator, on the basis of a membrane of aluminum nitride (AlN) deposited on a platinum/titanium metallic electrode. The substrate used, on which these layers rest, is made of silicon which has been etched by reactive plasma (RIE) to form a cavity. (H. Campanella, J. Esteve, E. Martincic, P. Nouet, A. Uranga, N. Barniol, IEEE SENSORS 2008).
The authors Pijolat et al. have shown the production of such resonators by transferring a thin film of LiNbO3 onto silicon substrate by virtue of a process based on direct bonding and mechanical thinning (M. Pijolat, S. Loubriat, S. Queste, D. Mercier, A. Reinhardt, E. Defaÿ, C. Deguet, L. Clavelier, H. Moriceau, M. Aïd, and S. Ballandras, Appl. Phys. Lett 95 (2009) 182106).
Other authors have proposed to fabricate an FBAR structure with a suspended layer of LiNbO3 and whose electrical characterizations at 200 MHz have been carried out: Y. Osugi, T. Yoshino, K. Suzuki and T. Hirai, IEEE 2007. Although the non-uniformity of the thickness over the wafer has a negative effect on the quality factor Q, the transferred layer of LiNbO3 possesses a coupling factor Kt2 close to that of the solid substrate.
Other types of resonators (including SAW resonators) are advantageously produced on suspended membranes.
Currently, two main techniques for transferring thin layers have already been proposed: a technique based on the implantation of light ions (typically hydrogen) and fracture at the level of the implanted zone, and the previously mentioned technique based on bonding and mechanical thinning. These techniques make it possible to transfer a monocrystalline layer onto a host substrate. These techniques are perfectly mastered on silicon allowing inter alia the industrial fabrication of SOI (Silicon On Insulator) dies.
The process of transfer by implantation/fracture is notably described in the article by M. Bruel: “Silicon on insulator material technology”, Electronic letters, 31 (14), p 1201-1202 (1995), it allows the production of SOI “Silicon On Insulator” substrates.
This process may be schematically summarized by the following four steps illustrated in FIG. 2:
Step 1: A donor substrate A, for example of silicon, is implanted with gaseous species (for example hydrogen and/or rare gases) to form a buried fragile zone, delimiting in this substrate the thin film to be transferred.
Step 2: The donor substrate is thereafter joined at the level of the previously defined thin film, for example by direct bonding (also called molecular bonding), with a receiving substrate B.
Step 3: A fracture step is thereafter obtained at the level of the buried fragile zone by means of a heat treatment optionally assisted by the application of mechanical stresses. One thus obtains on the one hand the thin film secured to the receiving substrate, and on the other hand the remnant of the donor substrate corresponding to the initial donor substrate A, peeled of the transferred thin film. The latter can then be recycled to carry out another transfer.
Step 4: Optionally, final treatments may be carried out, for example a high-temperature annealing to consolidate the bonding interface between the transferred thin film and the receiving substrate.
The thickness of the transferred thin film is directly related to the ion beam implantation energy. By way of example, the silicon thickness transferred can range from a few tens of nanometers to a few micrometers by using a conventional implanter (for which the implantation energy is typically less than 250 keV).
The transferred layers are uniform and homogeneous thickness-wise since they are defined by an implantation depth and not by mechanical thinning.
Document EP0741910 proposes to carry out a thin film transfer onto a substrate provided with cavities. The cavities are produced by photolithography and etching before the bonding step, which therefore adds an expensive step to the previously described process. Moreover, the achievable dimensions for the cavity are limited on account of the process.
Document EP0851465 proposes to locally debond a membrane from its support by implantation at the level of the bonding interface. In this case, the zone of strong implantation (around the bonding interface) undergoes significant damage that may cause local modification of the properties of the material.
Whereas today in the majority of components for MEMS “Micro-Electro-Mechanical Systems”, the layers of piezoelectric materials are produced by deposition techniques of PVD “Plasma Vapor Deposition” type and the layers to be made for these components exhibit thicknesses in a range of thicknesses lying between a few hundred nanometers and a micrometer, mastery of the fabrication of monocrystalline piezoelectric and electrostrictive layers for this range of thicknesses constitutes a significant technological hurdle.